This invention relates generally to acoustic devices and more particularly concerns dynamic loudspeakers.
It is presently known to fasten two cone loudspeakers together in either front-to-front or front-to-back relationship such that the moving systems of both speakers are not restricted and a small volume of air is contained between the cones of the loudspeakers. The voice coils of the two speakers are electrically connected in series or parallel and phased such that a current flowing in them causes the cone and voice coil in each to move in the same direction. When so configured, at frequencies where the wavelength of sound is large compared to the dimensions of the air chamber between the loudspeaker cones, the two speakers act together as one equivalent driver. Therefore, they may be modeled as an equivalent driver to predict their theoretical low frequency performance in any type of enclosure. Loudspeakers used in such an arrangement are commonly referred to as compound driver, constant pressure or isobaric loudspeakers.
Present practice is to use two identical loudspeakers with characteristics as close to each other as possible. The low frequency performance for a single driver are determined by the Thiele-Small parameters of free air resonance frequency Fs, effective motor strength Qts and equivalent volume compliance Vas. For two identical loudspeakers in a compound configuration measured as an equivalent driver, Fs and Qts remain the same as for a single loudspeaker while Vas is one half the single loudspeaker value. Such a compound driver has the same low frequency performance as a single driver but requires only half the enclosure volume and has half the efficiency of the single driver.
Despite these desirable features, compound driver loudspeakers have not been widely successful as factory built systems. High volume production of loudspeaker cabinets uses the techniques of miterfold construction, which results in an enclosure with all glued joints and no removable panels. Therefore, all loudspeaker drivers are front mounted on the outside surfaces of the cabinet. The only type of conventional compound driver system that lends itself to this arrangement uses two identical speakers mounted front-to-front and fastened to the outside of the enclosure. A front-to-back compound driver arrangement with identical drivers and connecting air chamber is not possible unless a removable panel is included in the enclosure. The use of removable panels in cabinets increases costs and negatively impacts the operating characteristics of the system if proper sealing of the removable panel is not achieved.
Therefore, the primary object of this invention is to provide a front-to-back compound driver loudspeaker system which does not require use of a removable panel in its enclosure. Another object of the invention is to provide a front-to-back compound driver-loudspeaker system which allows mounting of internal drivers in close proximity to the rear of an external driver to minimize the connecting air chamber volume while still providing straight on access to the mounting hardware for the internal drivers. Another object of this invention is to provide a front-to-back compound driver system in which the parameters of the internal drivers result in an equivalent single driver properly matching the external driver to optimize low frequency performance.